# std::defer_lock, std::try_to_lock, std::adopt_lock, std::defer_lock_t, std::try_to_lock_t, std::adopt_lock_t

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< cpp‎ | thread

Defined in header `<mutex>` |  |
struct defer_lock_t { explicit defer_lock_t() = default; }; | (1) | (since C++11)
constexpr std::defer_lock_t defer_lock {}; | (2) | (since C++11) (inline since C++17)
struct try_to_lock_t { explicit try_to_lock_t() = default; }; | (3) | (since C++11)
constexpr std::try_to_lock_t try_to_lock {}; | (4) | (since C++11) (inline since C++17)
struct adopt_lock_t { explicit adopt_lock_t() = default; }; | (5) | (since C++11)
constexpr std::adopt_lock_t adopt_lock {}; | (6) | (since C++11) (inline since C++17)

1,3,5) The empty class tag types `std::defer_lock_t`, `std::try_to_lock_t` and `std::adopt_lock_t` can be used in the constructor's parameter list for std::unique_lock and std::shared_lock to specify locking strategy.
2,4,6) The corresponding `std::defer_lock`, `std::try_to_lock` and `std::adopt_lock` instances of (1,3,5) can be passed to the constructors to indicate the type of locking strategy.

One of the constructors of the class template std::lock_guard only accepts the tag `std::adopt_lock`.

Type | Effect(s)
`defer_lock_t` | do not acquire ownership of the mutex
`try_to_lock_t` | try to acquire ownership of the mutex without blocking
`adopt_lock_t` | assume the calling thread already has ownership of the mutex

### Example

Run this code

    #include <iostream>
    #include <mutex>
    #include <thread>

    struct bank_account
    {
        explicit bank_account(int balance) : balance{balance} {}
        int balance;
        std::mutex m;
    };

    void transfer(bank_account& from, bank_account& to, int amount)
    {
        if (&from == &to) // avoid deadlock in case of self transfer
            return;

        // lock both mutexes without deadlock
        std::lock(from.m, to.m);
        // make sure both already-locked mutexes are unlocked at the end of scope
        std::lock_guard lock1{from.m, std::adopt_lock};
        std::lock_guard lock2{to.m, std::adopt_lock};

    // equivalent approach:
    //  std::unique_lock<std::mutex> lock1{from.m, std::defer_lock};
    //  std::unique_lock<std::mutex> lock2{to.m, std::defer_lock};
    //  std::lock(lock1, lock2);

        from.balance -= amount;
        to.balance += amount;
    }

    int main()
    {
        bank_account my_account{100};
        bank_account your_account{50};

        std::thread t1{transfer, std::ref(my_account), std::ref(your_account), 10};
        std::thread t2{transfer, std::ref(your_account), std::ref(my_account), 5};

        t1.join();
        t2.join();

        std::cout << "my_account.balance = " << my_account.balance << "\n"
                     "your_account.balance = " << your_account.balance << '\n';
    }

Output:

    my_account.balance = 95
    your_account.balance = 55

### See also

(constructor) | constructs a `lock_guard`, optionally locking the given mutex (public member function of `std::lock_guard<Mutex>`)
(constructor) | constructs a `unique_lock`, optionally locking (i.e., taking ownership of) the supplied mutex (public member function of `std::unique_lock<Mutex>`)

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